Nanovortex-Driven All-Dielectric Optical Diffusion Boosting and Sorting Concept for Lab-on-a-Chip Platforms.

The ever-growing field of microfluidics requires precise and flexible control over fluid flows at reduced scales. Current constraints demand a variety of controllable components to carry out several operations inside microchambers and microreactors. In this context, brand-new nanophotonic approaches can significantly enhance existing capabilities providing unique functionalities via finely tuned light-matter interactions. A concept is proposed, featuring dual on-chip functionality: boosted optically driven diffusion and nanoparticle sorting. High-index dielectric nanoantennae is specially designed to ensure strongly enhanced spin-orbit angular momentum transfer from a laser beam to the scattered field. Hence, subwavelength optical nanovortices emerge driving spiral motion of plasmonic nanoparticles via the interplay between curl-spin optical forces and radiation pressure. The nanovortex size is an order of magnitude smaller than that provided by conventional beam-based approaches. The nanoparticles mediate nanoconfined fluid motion enabling moving-part-free nanomixing inside a microchamber. Moreover, exploiting the nontrivial size dependence of the curled optical forces makes it possible to achieve precise nanoscale sorting of gold nanoparticles, demanded for on-chip separation and filtering. Altogether, a versatile platform is introduced for further miniaturization of moving-part-free, optically driven microfluidic chips for fast chemical analysis, emulsion preparation, or chemical gradient generation with light-controlled navigation of nanoparticles, viruses or biomolecules.

Biological Kerker Effect Boosts Light Collection Efficiency in Plants.

Being the polymorphs of calcium carbonate (CaCO3), vaterite and calcite have attracted a great deal of attention as promising biomaterials for drug delivery and tissue engineering applications. Furthermore, they are important biogenic minerals, enabling living organisms to reach specific functions. In nature, vaterite and calcite monocrystals typically form self-assembled polycrystal micro- and nanoparticles, also referred to as spherulites. Here, we demonstrate that alpine plants belonging to the Saxifraga genus can tailor light scattering channels and utilize multipole interference effect to improve light collection efficiency via producing CaCO3 polycrystal nanoparticles on the margins of their leaves. To provide a clear physical background behind this concept, we study optical properties of artificially synthesized vaterite nanospherulites and reveal the phenomenon of directional light scattering. Dark-field spectroscopy measurements are supported by a comprehensive numerical analysis, accounting for the complex microstructure of particles. We demonstrate the appearance of generalized Kerker condition, where several higher order multipoles interfere constructively in the forward direction, governing the interaction phenomenon. As a result, highly directive forward light scattering from vaterite nanospherulites is observed in the entire visible range. Furthermore, ex vivo studies of microstructure and optical properties of leaves for the alpine plants Saxifraga "Southside Seedling" and Saxifraga Paniculata Ria are performed and underline the importance of the Kerker effect for these living organisms. Our results pave the way for a bioinspired strategy of efficient light collection by self-assembled polycrystal CaCO3 nanoparticles via tailoring light propagation directly to the photosynthetic tissue with minimal losses to undesired scattering channels.